1. Field of the Invention:
The present invention relates to a position detection optical apparatus in which a light spot is obliquely projected onto an object surface to be examined and the reflected light from the object surface is detected to measure the position of the object surface. More particularly, the present invention relates to such optical apparatus for the detection of position in which the detection is carried out obliquely through a transparent plate covering the object surface to be examined.
2. Related Background Art
Projection optical systems for projecting a particularly shaped light spot onto an object surface and detection optical systems for detecting the reflected light from the object surface are all known in the art.
In this kind of the optical system it is most desirable to use such optical elements having an isotropic form relative to the optical axis of the optical system, that is, optical elements having a rotation-symmetrical configuration relative to the optical axis. By doing so, generally the most optimum construction can be obtained in view of the imageforming performance of the optical system. When the optical system contains one or more plane parallel transparent members such as window glass and prism block, the plane parallel member is usually disposed with its light incidence and exit surfaces being normal to the optical axis. In other words, it is a common practice in the art to arrange such plane parallel member in the position in which the normals to the respective planes of the member extend in parallel to the optical axis of the optical axis.
However, there may be such cases where the plane parallel member can not be disposed normally to the optical axis but it must be disposed inclined to the optical axis owing to some limitations or special conditions and man has to carry out the detection of position through the inclined plane parallel member.
For example, the following cases can be mentioned for it:
Man views obliquely an object surface through a window glass mounted extending in parallel to the object surface;
Man views an object contained in a sealed container obliquely and through a window glass; and
Man carries out the detection of focus to such an object contained in a container obliquely and through a window glass.
In these cases, the plane parallel member such as glass plate existing in the optical path is inclined. Therefore, if the light running along the optical path in which the plane parallel member is existing is not a collimated light beam, then there may be produced aberrations even at the object point on axis. The aberrations are asymmetrical to the optical axis. In this case, therefore, it is impossible to form a sharp and clear image of the object. The generation of such aberrations is more remarkable with increasing the inclination and thickness of the plane parallel member.
The reduction of optical performance of the optical system as mentioned above may be obviated by disposing the plane parallel member in a collimated light beam. However, to form the optical path of a collimated light beam, a long optical system is required, which is against the realization of the apparatus having a small and compact construction.
The above-mentioned problem is present also in the manufacture of semiconductor devices employing the so-called projection exposure apparatus.
As well known, in the manufacture of semiconductor devices with the projection exposure apparatus, there is the case where a wafer is contained in an air-tightly closed container filled with a particular gas. In this case, exposure to the wafer is carried out vertically from above through a glass window of the gas container. Under the condition, the focusing and alignment for the wafer must be carried out obliquely through the same window glass.
Many other limitations and conditions are known for this kind of the optical system under which we are obliged to dispose a plane parallel member (window glass) in a convergent beam or in a divergent beam (hereinafter referred to as non-collimated beam inclusively) or we can not form the desired collimated beam system. In these cases also the image-forming performance of the optical system is inevitably dropped down by the generation of asymmetric aberrations and we can not improve the accuracy of detection.